pre-ignition?

[simsalabim]

Hello all,

I am in the process of dialing in a recent 3.0L 81sc rebuild.

The motor runs well, except when the revs are low and the motor is under load (during moderately strong acceleration), it sounds like there is a bit of a "dieseling" noise coming from the exhaust area which disappears once hard acceleration ends.

The exhaust hangars are all tight and I do not believe there is anything rattling there. My current theories are:

1. pre-ignition
2. running rich
3. clog in the muffler (it has an older BB 2 in 1 out)

Any ideas on how I can check this out ? should I be overly concerned?

Thanks!

[cmcfaul]

pre ignition would acccure under those conditions if it's too lean, too hot, too much compresion, too much advance or too low octane.

Chris

73 911 E

[ossiblue]

You may want to add loose chain tensioners as a possibility to that list.

Should you be concerned? Absolutely, especially if it is pre-ignition or loose cam chains. Either of those conditions can lead to a catastrophic failure if left unattended. The best advice I can give is to find out what is causing the noise and do not drive the car unnecessarily until you do.

[simsalabim]

Thanks,

Is that what pre-ignition sounds like? If it is "mild" should I be concerned?

[ossiblue]

Originally Posted by simsalabim

Thanks,

Is that what pre-ignition sounds like? If it is "mild" should I be concerned?

Yes, you should be concerned not only because of the potential damage but because it can be remedied and the risk is needless. It's hard to confirm if what you experience is pre-ignition without riding along with you, but it certainly fits the description. Cmcfaul give you some great causes of pre-ignition. I'd suggest you start by eliminating each of them, one at a time, as a possible culprit.

[Reiver]

Pre ignition is hard to hear on our 'noisy' motors but sounds like ball bearings being shaken in a glass jar.
I'd be concerned (if it is det) because mild detonation can lead to serious detonation with a minor change in conditions.

[simsalabim]

Thanks for the advice.

I am thinking that first I will add some Octane booster. If it goes away, I will dial back my advance.

If not, I will re-check the mixture (although I would think it would be a rich situation as opposed to a too lean cause of pre-ignition?)

I'll leave the chain tensioners till last.

Sound like a good plan?

[lindy 911]

Rich mixture does not cause detonation (pre-ignition). A lean mixture will super heat parts of the piston top that will act as a spark plug totally out of time with your ignition wishes. Octane booster won't hurt but probably won't help either.

First thing is to make sure mixture is correct. Do you have CIS or carbs?
Where is the timing set now? Should be somewhere in the 26 degree advanced range all in.

Like has been said before, detonation is sometimes very hard to make out. It might be that what you're hearing is something all together different.

[simsalabim]

Thanks,

It's a CIS car. I have an Electromotive HPV-1 ignition system on it, and also just put in the Euro 9.8:1 PC set. I know that this warrants extra concern with respect to timing advance, but I was assured by my builder that the 29 all-in advance should not be and issue with such a high energy ignition.....?

As far as mixture goes, my tech manual says that for an 81 car with O2 sensor, the CO should be between .4 and .8 percent, taken with the O2 sensor disconnected and pre-cat (my car currently has a cat delete exhaust tip).

Do those numbers sound correct?

[Bob Kontak]

Try the "all in" timing at 25 and see what happens. I know it is a wimpy number, but the stock 81 is something like 24 max but with 87 fuel and 9.3:1 pistons.

At least you won't cook something while you troubleshoot.

[ClickClickBoom]

Pre-ignition and detonation are 2 separate issues. Given that you have increased the compression and are running a good ignition system, my bet is detonation. Jerry Woods told me 30-32 deg with a stock 2.7 was pushing the limit with todays fuel quality. Detonation sounds like someone pouring a shovel full of pea gravel on a corrugated metal roof, with an accompanying dramatic power loss. Detonation damage is cumulative, each event adds to the total damage. Reducing the advance and adding octane boost will help determine if it is detonation. Doubt it is pre-ignition.

[duncan1437]

Originally Posted by simsalabim

Hello all,

The motor runs well, except when the revs are low and the motor is under load (during moderately strong acceleration), it sounds like there is a bit of a "dieseling" noise coming from the exhaust area which disappears once hard acceleration ends.

Thanks!

i hestitate to mention this cause it seems too elementary, but is there any chance you're just lugging the engine (too much gas pedal at too low an rpm)?

[Reiver]

Originally Posted by simsalabim

Thanks,

It's a CIS car. I have an Electromotive HPV-1 ignition system on it, and also just put in the Euro 9.8:1 PC set. I know that this warrants extra concern with respect to timing advance, but I was assured by my builder that the 29 all-in advance should not be and issue with such a high energy ignition.....?

As far as mixture goes, my tech manual says that for an 81 car with O2 sensor, the CO should be between .4 and .8 percent, taken with the O2 sensor disconnected and pre-cat (my car currently has a cat delete exhaust tip).

Do those numbers sound correct?

I run a euro SC with the same compression figures and you need to run a fatter (richer) CO than those figures. I run a 3.0 at idle (afr 13.38) and when at speed under load and AFR of 14.5-14.7 (.2) which is close to ideal.
My engine is stock with SSI's and no cat which wouldn't make much dif anyway.
I have an on board AFR guage in place of the clock to monitor the afr.
I run in 100 plus degree weather often with no overheating (195-200 summer avg) on 91 Octane (only thing avail in Az).
The richer fuel setting also assists in that cooling.

[simsalabim]

I also run in Arizona.

Good food for thought here and I thank all. As far as the possibility of simply lugging the engine, would that create the dieseling noise I described (it sounds like scattershot pinging)? It does not seem to happen when I hard accelerate from say 3,000 rpm....

Also, by adding the Octane booster and/or dialing back the advance, which possibility will be tested for, pre-ignition or detonation?

[ossiblue]

Originally Posted by simsalabim

I also run in Arizona.

Good food for thought here and I thank all. As far as the possibility of simply lugging the engine, would that create the dieseling noise I described (it sounds like scattershot pinging)? It does not seem to happen when I hard accelerate from say 3,000 rpm....

Also, by adding the Octane booster and/or dialing back the advance, which possibility will be tested for, pre-ignition or detonation?

I have to agree with ClickClickBoom and say what you are likely experiencing is detonation rather than pre-ignition. The difference between the two can be described as one is "too fast" (detonation) and the other is "too soon" (pre ignition).

Fuel is supposed to begin a burn (not explode) from a moment before the piston reaches the top of its stroke until the piston reaches its top, where the hot, expanding gas forces it back down. Detonation happens when the fuel mixture detonates or explodes (burns too fast) and expells its full energy on a piston that is moving upward towards it, causing a "bang" from its impact on the moving piston and its assembly. This is caused, as has been posted, by too hot of conditions within the cylinder (from a too lean a mixture), improper timing, or too low of octane. Adding octane booster or dialing back the timing can reduce the chances of detonation as can a fuel mix enrichment.

[simsalabim]

OK then, I will do some experimenting with the timing, Octane and mixture.

I've got about 500 miles on it since the build. How much damage might have been done? Any ways to check once I figure it out?

[Raceboy]

Pre-ignition is not audible at all, what you hear is detonation. And even though engines can withstand SOME detonation, pre-ignition usually burns a hole through a piston in a matter of seconds.

[T77911S]

too much advance. good advice to reduce it to around 26 degrees.

check the ground strap on the plugs. there should be an annealing mark or some kind of line on the strap that will indicate if your timing is good. it should be at the bend in the strap.

[Reiver]

Originally Posted by simsalabim

OK then, I will do some experimenting with the timing, Octane and mixture.

I've got about 500 miles on it since the build. How much damage might have been done? Any ways to check once I figure it out?

You are probably all right if you get on it quickly but you should do so. At our compression detonation in our 3.0 's leads to broken rings and lower piston skirt damage. If you don't find bits in the oil you should be fine and I suspect you'd have more issues anyway.
Dial it back and use a booster.

[duncan1437]

Originally Posted by simsalabim

I also run in Arizona.

Good food for thought here and I thank all. As far as the possibility of simply lugging the engine, would that create the dieseling noise I described (it sounds like scattershot pinging)? It does not seem to happen when I hard accelerate from say 3,000 rpm....

about the lugging, the noise i'm familiar with is a chugging, bucking, stumbling sound, rather than bbs in a coffee can.

since lugging only happens in 2nd gear and up, you could check the lugging theory by briskly accelerating (like you did before) from a standing start in 1st gear. if it DOESNT diesel below 3000 rpm, i'd guess your earlier experience below 3000 rpm was just lugging. (i'm assuming your earlier runs were in 2nd gear and up.)

about your ignition timing, there's a good discussion here

SC Ignition Timing Question

the discussion is about a 1983 SC, but i understand the timing is the same for the 1980-83 USA cars.